Aluminum alloys containing lithium (Li) are of great interest in the aerospace industry due to the possibility of producing structural components having a high strength/weight ratio. Typically, these alloys contain approximately 0.5 percent to 3.0 percent lithium by weight.
Metal manufacturers subject these alloys to heat treatment including solution heat treatment homogenization and annealing. For solution heat treatment, temperatures employed usually range 475.degree.-575.degree. C. At these elevated temperatures there can be a significant lithium content loss. In the case of sheets having a thin thickness, the lithium loss at these temperatures can be extensive during normal heat treatment times due to reaction of the lithium in the sheet material and the oxygen in a furnace atmosphere. The prior art includes attempts to create a furnace atmosphere that will reduce the rate of reaction. However, there are two primary considerations in a heat treatment process of aluminum/lithium alloys as regards the relationship between the lithium content and the furnace atmosphere. These include:
a) loss of lithium from the alloy, and PA1 b) formation of reaction by-products which may penetrate intergrain boundaries.
In order to better appreciate the mechanism of lithium loss, one must consider the diffusion of lithium atoms toward the surface of the material where reaction with the atmosphere takes place. The result of this diffusion is a near-surface region of the material where the concentration of lithium is less than the desired lithium concentration in the alloy. This lithium depleted region will not have the properties required of the alloy, i.e. it will be mechanically weaker, it may have different fatigue resistance corrosion resistance, etc. In addition, the result of this atomic migration is the generation of lithium atom vacancies. It has been noticed that a tendency develops for these vacancies to agglomerate thereby causing voids or "pores" in the material. The lack of adequate lithium in the near-surface material and the presence of pores, of course, weakens the metal.
The prior art has recognized that, in the latter case of penetrating by-products, the adverse effects of reaction products (in relation to the weight of such products) increases in line with the increase of volume due to the formation of such products. The penetration of the intergrain boundaries is highly undesirable in thin alloy sheet materials because of the severe loss of alloy integrity.
Another problem arising from heat treatment of aluminum-lithium (Al-Li) alloys is the loss of lithium near the surface which weakens the material and leaves the material near the surface with a high degree of porosity. From a structural point of view, such porosity is undesirable.
In published U.K. Patent Application 2,137,666 A, filed Mar. 27, 1984, and published Oct. 10, 1984, an atmosphere is proposed that is essentially a CO.sub.2 atmosphere containing a definite moisture content which can be employed as at atmosphere in the heat treatment of Al-Li alloys because the introduction of small amounts of oxygen, nitrogen, and water vapor from an ambient atmosphere would not be especially deleterious in relation to the rate of attack on the Li content of the alloy. According to the published U.K. application, a heat treatment of an Al-Li alloy is carried out in an atmosphere consisting essentially of carbon dioxide having a water content controlled to be in the range of 4 to 250 Torr or even higher (about -0.6 to 31 percent by weight). However, a preferable range of 10 to 50 Torr is prescribed since it is easily achieved.